This invention relates to the remediation of nitrate contaminated aquifers, such as in agricultural areas or at airports, and in particular to a method and apparatus for providing such remediation in situ.
Concern about elevated concentrations of nitrate in drinking water is growing, especially in rural areas where farming activities as well as the use of nitrate-rich fertilizers increase levels of nitrate in groundwater. Another source of nitrate in groundwater, stems from the use of urea on airport runways as a deicing agent. In this later case, urea percolation in the upper layer of soil is followed by biologically assisted hydrolysis of urea into ammonium. The aerobic nitrifying bacteria (ammonium- and nitrite-consumers) complete the transformation into nitrate. In both cases nitrates are carried with water seeping into the deeper aquifer, where they may accumulate in an aerobic environment which lacks a carbon source.
One prior art approach to the problem is described in a paper by (A. Mohseni-Bandpi, D. J. Elliot, A. Momeny-Mazdeh. Denitrification of groundwater using acetic acid as a carbon source, Water Science Technology, v 40, pp. 53-59.). This reference describes the treatment of groundwater in an aboveground bioreactor.
U.S. Pat. No. 5,482,630, which issued on Jan. 9, 1996 to Phillip G. Lee et al., describes a process and system for anaerobic denitrification of nitrate in a fluid medium, also in an aboveground bioreactor. The controlled addition of a carbon source is also described. Process control is based upon controlling the addition rate of the carbon source and residence time, by monitoring the oxidation-reduction potential of the process.
U.S. Pat. No. 4,683,064 of Hallberg and Martinell, dated Jul. 28, 1987, discloses an in situ approach to decreasing the nitrate content in ground water. Denitrifying organisms and/or a substrate therefore are introduced by means of a number of injection wells arranged around one or more remediated water extraction wells. The wells are arranged such that a treatment zone is remote from the extraction wells. More specifically, the reference system comprises a complex system of oxidation and reduction wells arranged in two concentric circles around an extraction well located in the centre. For denitrification, the reference patent method requires the simultaneous introduction of water containing denitrification organisms and/or a substrate therefor into some of the injection wells, while water is drawn from adjacent injection wells, to create between the injection wells an annular denitrification zone which is spaced from the extraction well. The groundwater to be treated is passed through this denitrification zone to the pure water extraction well and withdrawn therefrom. It will be appreciated that the complexity and cost of a system including such a large number of wells would be prohibitive.
This technology is aimed at bioremediation of nitrate contaminated groundwater. Certain conditions (high dissolved oxygen concentration and low content of dissolved organic compounds) combined with a nitrate source run-off may result in nitrate accumulation in groundwater. While natural attenuation (denitrification) is limited under these conditions, often indigenous denitrifying bacteria could be found in groundwater and soil samples. Therefore, activity of indigenous denitrifying populations can be improved through addition of a carbon source to groundwater. Following groundwater amendment in a carbon source, both aerobic and anaerobic bacteria proliferate. The aerobes consume oxygen thus creating anaerobic conditions required for biological denitrification.
According to one aspect of the invention, the remediation procedure involves a combination of several steps described below, which when combined form a unique process for the in situ remediation of nitrate contaminated groundwater. Following hydrogeological and geophysical site characterization, a remediation sequence is carried out as described below:
1) Microbial characterization of groundwater and soil to enumerate indigenous denitrifying bacteria. The enumeration procedure uses a mixture of sulfanilic acid and xcex1-naphthylamine to visualize denitrifying colonies.
2) Laboratory batch denitrification tests under carbon source amended conditions to evaluate feasibility of substrate stimulated denitrification.
3) Optimization of the well setup which consists of a network of injection and extraction wells using a mathematical model. Both the distance between the wells and the number of injection wells is optimized.
4) Groundwater remediation using a mobile groundwater treatment apparatus which provides water pumping from an extraction well, carbon source addition, and water return to several injection wells.
According to another aspect of the invention, a method for biological denitrification of groundwater in situ in an aquifer under substantially anaerobic conditions is provided, comprising
(a) providing a network, including an extraction well for removal of groundwater from the aquifer and a plurality of injection wells for returning groundwater to the aquifer, the injection wells being arranged in a line substantially perpendicular to the direction of groundwater flow, the extraction well being located downstream or upstream of the line of injection wells, and the spacing of the wells and the number of injection wells being determined from the solution to the following optimization(minimization) problem                     J        =                                                            C                p                            +                              nC                1                                      W                    →          min                                    (        1        )            
where J is the cost of treatment, CP and CI are the costs of extraction and injection wells, respectively; n is the number of injection wells, and W is the distance across the protected area, and wherein the water stream is evenly distributed between injection wells, according to the relationship:
Fi=Fe/n=const xe2x80x83xe2x80x83(2) 
where F is the water flow rate (exe2x80x94extraction well, ixe2x80x94injection well) and n is the number of injection wells,
(b) removing groundwater from the aquifer by the extraction well,
(c) adding a carbon source to the groundwater in a controlled manner,
(d) returning the carbon source amended groundwater to the aquifer by the injection wells in even distribution, the water flow rate being controlled, and
(e) monitoring the nitrate concentration of the groundwater removed from the extraction well, a decrease in nitrate concentration being indicative of denitrification.
According to yet another aspect of the invention, an apparatus for biological denitrification of groundwater in situ in an aquifer under substantially anaerobic conditions is provided, comprising
(a) network means, including an extraction well for removal of groundwater from the aquifer and a plurality of injection wells for returning groundwater to the aquifer, the injection wells being arranged in a line substantially perpendicular to the direction of groundwater flow, the extraction well being located downstream or upstream of the line of injection wells, and the spacing of the wells and the number of injection wells being determined from the solution of the following optimization(minimization) problem                     J        =                                                            C                p                            +                              nC                1                                      W                    →          min                                    (        1        )            
where J is the cost of treatment, CP and CI are the costs of extraction and injection wells, respectively; n is the number of injection wells, and W is the distance across the protected area, and wherein the water stream is assumed to be evenly distributed between injection wells, according to the relationship:
Fi=Fe/n=const xe2x80x83xe2x80x83(2) 
where F is the water flow rate (exe2x80x94extraction well, ixe2x80x94injection well) and n is the number of injection wells, means for removing groundwater from the aquifer by the extraction well,
(b) means for adding a carbon source to the groundwater,
(c) means for returning the carbon source amended groundwater to the aquifer by the injection in even distribution,
(d) means for controlling water flow rate and addition of carbon source, and
(e) means for monitoring the nitrate concentration of the groundwater removed from the extraction well, a decrease in nitrate concentration being indicative of denitrification of the groundwater.